Elliptical gears are meshed in pairs for use in devices such as flow meters. Such elliptical gears have different major and minor axis dimensions, and may be meshed together so that each gear rotates about the elliptical gear center. For such gears to mesh continually during rotation, the gears pass through a position every 90.degree. wherein the major axes of the gears are perpendicular to each other. For instance, in an initial position, the major axis of a first paired gear is coincident with a line connecting the centers of the two gears, and the major axis of a second paired gear is perpendicular to the center line. When the gears have rotated 90.degree., the major axis of the second gear is coincident with the center line, and the major axis of the first gear is perpendicular to the line.
While a pair of true elliptical gears may mesh snugly when the gears are in the initial 0.degree. position, the meshing becomes loose when the true elliptical gears are rotated away from the initial orientation. The loose meshing is due to a gap that forms between the true elliptical gears four times during every full rotation of the gears As the gears rotate 30.degree.-70.degree., 120.degree.-160.degree., 210.degree.-250.degree., and 300.degree.-340.degree. away from the initial position, the gap causes loose meshing by pulling the meshing teeth slightly apart. The gap reaches maximum values at 45.degree., 135.degree., 225.degree., and 315.degree. rotation.
Loose meshing is undesirable because it inhibits the smooth rotation of the gears. Moreover, fluid may leak between the loosely meshed teeth, tending to degrade the effectiveness of true elliptical gears in flow meters and pumps.
Elliptical gears have another disadvantage in being impractical to mechanically hob. Mechanical hobbing is a simple and effective means of producing circular gears. In such hobbing, a circular gear blank is rotated about a fixed center point, while a hobbing rack with a series of cutting teeth moves transversely over the edge of the gear blank to cut teeth into the blank. Such mechanical hobbing produces precise involute-shaped gear teeth on the circular gear that snugly mesh with involute teeth on a meshed gear.
The application of mechanical hobbing to elliptical gear is problematic due to the varying radii of an ellipse. The variable radii would require the center of an elliptical gear blank to be precisely moved toward and away from the hobbing rack as the gear blank is rotated in order to properly cut teeth. Such movement would require an expensive and complicated control system.